Centrifuge deviation sensing switching mechanism



J. BLUM July 14, 1959 CENTRIFUGE DEVIATION SENSING -SWITCHING MECHANISM l Filed June 2o. 195e 2 Sheets-Sheet 1 v INVENTOR ,70.55F S20/w ATTORNEY J. BLUM CENTRIFUGE DEVIATION S-ENSING SWITCHING MECHANISM Filed June 2o. 195:3

2 Sheets-Sheet 2 N mm,

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u w n ATTORNEY United States Patent O CENTRIFUGE DEVIATION SENSING SWITCHING MECHAN ISM Josef Blum, Norwalk, Conn., assignor to Ivan Sorvall, Inc., Norwalk, Conn., a corporation of New York Application .lune 20, 1958, Serial No. 743,303

14 Claims. (Cl. 200-61.45)

This invention relates to electrical switches, and more particularly to switching arrangements for use in controlling the operation of centrifuges.

In the operation of centrifuging apparatus, there is encountered the problem of dealing with oscillations of the centrifuge rotor when it departs from its normal rotating axis due to asymmetrical loading of the rotor receptacles, or when, in the case of continuous centrifuging apparatus, rotor imbalance develops due `to faulty or uneven distribution of suspensions to the various receptacles. Such rotor imbalance or asyrrnnetrical loading produces undesired deviations of the rapidly spinning rotor from its normal axis which should be stopped promptly in order to prevent damage to the centrifuge, and to safeguard the operator of the apparatus.

One rneans for sensing the deviation of the rapidly spinning rotor from its normal axis is the provision of a microswitch which is arranged to be operative, when a predetermined critical deviation is reached to cut oif the electric current to the motor connected to the rotor drive shaft. Since it is a characteristic of a rapidly spinning centrifuge rotor to repeat its oscillations a great number of times, the juxtaposition of the microswitch actuating button to the rotor or its rotor shaft would involve virtually countless off-on actuations of the switch, thereby defeating its purpose and subjecting it to excessive wear.

In order to produce a single actuation upon the microswitch whereby it can perform its function properly and its life can be prolonged, there are provided herein, deviation sensing switch mechanisms which are operative only once by the rapidly spinning rotor shaft to cause the microswitch to open the drive motor circuit at a predetermined degree of deviation and to keep said circuit open, either upon the beginning of the centrifuge run, or upon an imbalance condition developing during the centrifuge run.

When the motor drive circuit of a centrifuge is opened, the amplitude of deviation of an unbalanced rotor usually increases as the rotor slows down. Since such deviations would otherwise be repeated upon the sensing switch mechanism, there is provided an overtravel arrangement whereby continued impingements of ever increasing amplitude upon said mechanism will not produce any further actuation of the microswitch contact button. Thus the life of a microswitch is greatly prolonged since its contacting elements are not subjected to undue wear.

Still other objects and advantages of my invention Will be apparent from the specification.

The features of novelty which I believe to be characteristic of any invention are set forth herein and will best be understood, both as to their fundamental principles and as to their particular embodiments, by reference to the specification and accompanying drawings, in which:

Figure l is a fragmentary View of a centrifuge apparatus, taken in vertical, central cross section, some parts being shown in phantom outline and some parts being shown in elevation; l

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Fig. 2 is a top view of the apparatus shown in Fig. 1, taken on line 2 2 of Fig. 1, some parts being shown in section and some parts being shown in phantom outline;

Fig. 3 is a fragmentary view of a centrifuge apparatus showing an alternative embodiment of the switch shown in Fig. l, together with a reset mechanism for remotely controlling said switch;

Fig. 4 is a top view of the apparatus shown in Fig. 3, some parts being shown in phantom outline and some parts being omitted; and

Fig. 5 is a greatly enlarged, partial central section viewof a portion of the switch control components shown in Fig. 3.

Referring now to the drawings in detail, the apparatus of the present invention is utilized in conjunction with a centrifuge which comprises a horizontal frame 11 having a plurality of legs, not shown, for positioning the apparatus on a table or other suitable platform. Frame 11, only a fragment of which is shown in Fig. l, has a circular array of spaced apart, upwardly extending cups 12, only one of which is shown, whose recesses 13 accommodate the lower portions of respective cylindrical resilient cushions 14 made of rubber or the like.

Resting upon the upper ends of cushions 14 is a spider 15 which has a circular array of vertical apertures 16 coaxially aligned with respective central apertures 17 of cushions 14. A stud 18 extends through each pair of apertures 16 and 17.

Connected to frame 11 is a circular spider ring 22, the upper portion of which surrounds spider 15. Since all portions of ring 22 are spaced apart radially from all adjacent peripheral portions of spider 15, a limited amount of lateral vibration or oscillation of spider 15 in respect of spider 22 is permitted as the centrifuge rotor rotates about its shaft. and positioned between the legs thereof is an electric motor, not shown, which provides power for operating ythe centrifuge rotor shaft.

Formed integrally with spider 15 is an upwardly extending cylindrical shaft housing 32 which contains a pair of spacer apart ball bearings 33 rotatably supporting a rotor drive shaft 34 coupled by suitable means toA the motor drive shaft, not shown.

Connected to the upper end of shaft 34 by means of nut 35 is a circular centrifuge rotor 36 made of aluminum, aluminum alloy, steel or the like, and having a plurality of spaced apart recesses 37 arrayed radially and uniformly therein. Recesses 37 are positioned at an angle in respect of the vertical axis of the rotor as in the conventional angle centrifuge. The angle of inclination of said recesses 37 may lie between O90 from the vertical, depending upon the centrifuging conditions that are desired. Each recess 37 accommodates a suitable test tube 38 for the collection of precipitates or sediments resulting from centrifugation.

Spider ring 22 has a radially extending bracket 39 which is arranged to support the switching and control elements of the present invention. Connected to the bottom surface of bracket 39 is a microswitch frame 42 secured thereto by means of a screw 43. Securely supported in microswitch frame 42 is a microswitch 44 of the conventional type, said microswitch having an actuating button 45 which is normally urged outwardly from said switch to a limited unactuated position by means of a spring or the like, not shown. Microswitch 44 operates by way of suitable leads, not shown, to turn on and off the electric motor which drives the centrifuge shaft.

Connected to the side of microswitch 44 is a hinge block 46 secured thereto by means of screw 47. Pin

48 on hinge block 46 pivotably supports an upwardly'A Connected rigidly to frame 11- asaaoas extending trip lever 419. Connected to the left side of trip lever 49 is a trip lever spring 50, the curved free end of which resiliently engages a side wall of microswitch 44 and normally yurges said trip lever to the right, as shown in Fig. l.

Bracket 39 has a pair of apertures Si and StA which are separated by partition 52. Both partition 52 and ring 22 each have axially aligned apertures which slidably accommodate tripper rod 53. Under the action of spring 50, the upper end of trip lever 49 normally urges tripper rod 53 to the right, whereby its inner end is maintained against the side of spider l5.

Mounted in bracket 39 and positioned substantially at right angles to and below tripper rod 53, between ring 22 and partition 52, is a reset lever shaft 55 extending across aperture SllA.

An L-shaped reset lever, generally designated 56, has a leg 57 whose bifurcated toe 58 pivotably engages reset lever shaftl 5S. Reset lever 56 has a horizontal arm 59 which extends over bracket 39 and is normally urged upward by means of reset lever spring 6l, positioned vertically in aperture 62 bored in bracket 39.

Leg 57 of reset lever Se has a lateral aperture 63 through which tripper rod 53 extends for reciprocable movement. When the outer end of arm S9 is depressed against the action of spring 6l, the inner walls of aperture 63 become aligned substantially parallel to the outer walls of rod 53 whereby rod 53 is free to be .urged by the action of lever 49 and spring Si) to the right so that its right end abuts and is normally maintained against the peripheral edge of spider l5. When arm 59 is urged upwardly by the action of spring el, the slight pivoting action of leg 57 causes the inner walls of aperture 63 to be positioned at a slight angle or skewed relative to the outer walls of rod 53 thereby producing a frictional engagement therebetween.

r[his frictional engagement is arranged to be just sufiicient to maintain rod 53 in position against the action of spring S on lever i9 while at the same time it is insufficient to prevent its movement to the left under the vibratory action of spider l should said spider deviate from its normal position under the action of the rapidly spinning unbalanced rotor 36. Thus, as the spider oscillates at any time during the operation of the centrifuge, its deviations from its normal axis will cause g rod 53 to move to the left against the action of spring 50. To the extent that rod 53 is moved to the left, leg 57 is in such a position as to prevent the return of rod 53 to an abutting position against spider 15 and therefore rod 53 remains fixed in position.

As the shaft of rotor 36, and accordingly, spider l5, deviates from the vertical either on starting up or slowing down, rod 53 will be urged outwardly to the extent of said deviation; upon succeeding deviations of ever increasing amplitude, rod 53 will be repositioned at further successive outward positions relative to the normal vertical axis of the rotor shaft and will maintain those successive positions by virtue of the skewed relationship of aperture 63 relative to the surface of rod 53. When the deviation of the rotor shaft reaches a predetermined danger point either during starting or running operations, lever 49 is caused to depress button 45 of microswitch 44 into an actuating position whereby current to the motor operating shaft 34 is cut oif and the rotor is permitted to stop.

As an unbalanced rotor continues its deceleration, the oscillations from the vertical gain in amplitude thereby causing rod 53 to move further to the left (Fig. l), and to cause the further pivotal motion of lever 49. Since button 45 has already been depressed to the open circuit position by lever i9 when the critical deviation has been reached, it will .remain depressed to preserve the open circuit condition of microswitch dei notwithstanding the fact that spider llS continues its rapid oscillations during the more or less prolonged deceleration of the rotor.

With oscillations of increasing amplitude, after button 45 has been depressed to the open circuit position, the further movement of lever 49 will cause said button to move beyond the open circuit position but nevertheless said lever is prevented by the mechanism described hereinabove from releasing button 455 whereby repeated impingements thereof are prevented. By this means, button 45 and the contacting elements that it controls within microswitch 4d are protected from the extremely large number of impulses it would otherwise receive if the arrangement of rod 53, aperture o3 in leg 57, and lever 49 were not provided. In other words, this mechanism provides for the multitudinous and repeated overtravel of the switch operating mechanism after the switch has been put into the open circuit condition and its internal contacting elements are protected from unnecessary impingements that would otherwise obtain without the protective mechanism provided herein.

The angle of lever 49 relative to button 45', the length of said lever, and the position and length of rod S3, are coordinated to provide for the actuation of switch button 45 at a predetermined critical deviation of the spinning rotor from its normal axis. If the pre-critical deviation of the rotor is insuliicient to actuate the microswitch, the actuating lever 49 merely remains in a partially displaced position until the critical deviation is reached, at which time the lever i9 actuates the switch button.

In starting the centrifuge and during acceleration of the rotor, the problems of deviation of the rotor from its normal vertical axis are usually minimal unless the rotor happens to be unbalanced to begin with, by reason of the asymmetrical distribution of materials amongst the test tubes or receptacles 38. lf such a condition obtains, a critical rotor deviation will actuate the microswitch button, and open the electrical circuit to the centrifuge motor, thereby permitting the rotor to decelerate to a stop so that correction of the unbalanced condition of the rotor may be made.

In continuous centrifugation, as is described, for cxample, in United States Patent No. 2,834,541, there is the possibility that an imbalance in the rotor may develop because of an improper distribution to, or asymmetrical retention of materials amongst, the rotor receptacles; and such an imbalance may cause the spinning rotor to deviate from its normal vertical axis. A certin amount of ydeviation of the rotor can be tolerated without necessarily stopping the centrifuging process, but when the deviation becomes critical, then the arrangement described hereinabove will promptly open the circuit to the drive shaft motor, thereby permitting the rotor to decelerate to a stop so that the rotor imbalance can be corrected.

In a decelerating unbalanced rotor whose rotational frequency decreases, the amplitude of its deviation from its normal rotor axis increases. Thus if the switch button 45 were impinged upon directly by spider l5, the repeated impingements by said contact element due to the continued deviating oscillations of the decelerating rotor would produce an enormous number of actuations of said switch button and consequently upon the switching mechanism within the switch body, thereby subjecting the switch to actuations in excess of its normal rating and greatly reducing its life expectancy.

This can be better understood by considering that a rotor may rotate up to about 18,000 revolutions per minute. Should the rotor be unbalanced and deviate from its normal vertical axis, the number of oscillations of the rotor may be as much as 18,000 vibrations per minute. If the unbalanced rotor should take seven minutes to come to a stop, then the possible number of oscillations from the normal axis may be as much as 45,00() per minute; and that many direct impingements upon the switch button of the microswitch would clearly reduce its life expectancy to the extent that microswitches would have to be replaced almost daily, considering the fact that a microswitch is rated for a life span on the average of 1,000,000 actuations. It becomes evident that no switch can be utilized for the purpose of opening a motor circuit when unbalanced centrifuge rotors are to be dealt with unless some means are provided to relieve the switch of such excessive wear.

Consequently, the parts described hereinabove are preset to cause the actuation of microswitch 44 at a predetermined amplitude of deviation of the centrifuge rotor from its normal axis of rotation. After having actuated switch button 45, lever arm 49 continues to be operated upon by lthe oscillating rotor through spider and rod 53 if said udeviation increases in amplitude. Such increase in amplitude, however, does not subject switch button 45 `to any further on-off actuations since rod 53 is frictionally retained in each successive displacement by the skewed walls of aperture 63 of reset lever 56 under the action of spring 61.

After rotor 36 has come to rest, arm 59 of lever 56 may be depressed manually against the action of spring 61, thereby relieving the frictional engagement between aperture 63 and rod 53, whereby the latter is caused to return to an abutting position against spider 15 under the action of spring 50 and lever 49. By this action, selfreturning button 45 returns to the right to an unactuated position whereby the contacts within switch 44 are closed again and the drive motor is restarted. Now the rotor imbalance detecting and switch combination is ready for use again in the next succeeding centrifuging run.

By the means described hereinabove, switch 44 is effectively insulated from continued on-oif actuations due to multitudinous rotor deviations, and hence its life expectancy is vastly increased. In practice, switch button 45 and the contacting elements within microswitch 44 are actuated only once for each time that a critical rotor deviation is sensed by the combination of the cooperating elements described herein.

Another embodiment of the present invention is shown in Figs. 3, 4 and 5 of the drawings, wherein shaft housing 32 of rotor 36 is positioned within an aperture 71 of a stationary frame 72, said aperture having a sufciently large diameter to permit some lateral movement of said shaft housing therein. Connected to the bottom surface of frame 72 by means of screws 73 is a downwardly extending bracket 74. Connected by suitable means to one side of bracket 74 is a microswitch 75 having a switch button 76 (Fig. 5), which is normally urged upwardly from said switch to a limited unactuated position by means of a spring or the like, not shown.

Connected to the top of microswitch box '75 and surrounding switch button 76 is a cylindrical fitting 77 which longitudinally and slidably accommodates a vertical actuating button 78, `said button having at its lower end an annular ange 79. The interior of fitting '77 has an annular shoulder 81 which cooperates with flange 79 to limit the upward movement of actuator button 7 8.

Actuator button 78 has a central annular recess 82 in the bottom thereof, which accommodates the upper portion of a coiled spring 83, the bottom of which bears against the top of switch box 75 and normally urges button 78 upwardly to the limit permitted by ange 79 abutting shoulder 81. The bottom surface of the central stem 84 of actuating button 78 is adapted to engage and press down upon microswitch button 76 when actuator button 78 is depressed to a predetermined distance.

Mounted -around litting 77 is bracket 85 which is secured thereto between rings 86 and 87 threadably engaging the threaded surface of fitting 77. Bracket 85 has a pair of upwardly extending lugs 88 which cooperate with lug 89 of reset lever 90. These parts extend at least partially into a suitable aperture 91 in frame 72. Lugs 88 and 89 are pivotally connected to each other by means of pin 92. Reset lever 90 has an aperture 93 through which actuator button 78 extends for reciprocable movement. Surrounding actuator button 78 is a coiled spring 94, one end of which bears upon the top surface of ring 86, and the other end of which bears against the bottom surface of lever 90, said spring normally urging upwardly that portion of said lever which surrounds said actuator button 78.

Mounted fast on shaft housing 32 of the centrifuge is a support ring to which is connected an unbalance arm 96 by means of suitable screws or the like. Unbalance arm 96 curves downwardly as shown in Fig. 3, and its outer end has a threaded aperture which accommodates vertical set screw 97, the lower end of which can be adjustably located in a predetermined position relative to the top of actuator button 78. Nut 98 engages the threaded stem of screw 97 to secure the latter in its requisite position.

Since the lower end of set screw 97 is located directly over the top of actuator button 78, any angular deviation of shaft housing 32 from the vertical will cause said screw to bear down upon and depress said button to the extent of such deviation.

The operation of the switch actuating elements described in Figs. 3, 4 and 5 is comparable to that described in connection with Figs. 1 and 2. The walls of aperture 93 in lever 91 are arrayed in such a manner that when the actuator button 78 is depressed, the action of spring 94 will cause said aperture walls to engage and retain said button in the position to which it has been depressed by the action of screw 97 as shaft housing 32 deviates from its normal rotating axis.

The various parts are airanged relative to each other whereby upon critical deviation of shaft housing 32 from its normal rotating axis, stem 84 of button 78 will depress microswitch button 76 to open the electrical circuit to the motor driving the centrifuge shaft. Upon further ampliiied deviation of housing 32 from its normal rotating axis, screw 97 will further depress actuator button 78 against the action of spring 83, causing said actuator button 78 to overtravel beyond the point of actuation of microswitch button 76. In order to prevent multinidinously repeated impingements of stem 84 upon microswitch button 76, the walls of aperture 93, being skewed relative to the walls of button 78, will retain said button in each overtravelled position against the action of spring 83, whereby the microswitch button 76 will simply remain in the depressed open circuit condition so that the contacting elements within switch 75 will not be subjected to undue wear.

After the vcentrifuge rotor 36 has come to rest, and before a succeeding centrifuging run is begun, the outer end of lever 90 is lifted, whereby the walls of aperture 93 release actuator button 78 which is then urged upwardly by spring 83. Upon the upward movement of button 78, switch button 76 moves upwardly to its normal unactuated position by the internal mechanisms of switch 75, which are well known in the art.

Although lever 90 may be lifted manually, if the centrifuge is mounted in an open frame whereby said lever is readily accessible, in some centrifuging apparatus, it is necessary to surround the apparatus by an enclosed shield for protective purposes or the like, in which case lever 90 is not accessible for manual resetting. Consequently, there is provided a remote `control resetting system which can be operated externally of the shield.

Mounted on the front panel 101 of the centrifuging apparatus is a plunger assembly, generally designated 102, which comprises a cylindrical fitting 103 which is threadably secured to a portion of a bushing 104 which in turn is threadably secured into a suitable aperture in panel 101. Positioned within the central aperture of bushing 104 is a reset button 105 which is movable longitudinally therein. Button 105 has a ange 106 which cooperates With an annular internal shoulder 107 in bushing 104 to limit the outward movement of button 105 relative to bushing 104. Connected to assembly 102 is a journal 108 within which a plunger 109 is movable longitudinally at one end thereof. Connected to the other end of journal 108 is a cable sheath Illlil which is secured within journal 108 by means of set screw M2. Positioned between the inner end of cable sheath lll and plunger 109 is a spring 113 which normally urges plunger M9 outwardly toward the inner end of reset button 1195.

Positioned coaxially within sheath lll is a longitudinally movable reset shaft lllli, one end of which is connected fast to plunger 109. The other end of sheath MI extends through a suitable aperture in reset block 115 connected to bracket 74, and is secured therein by a suitable screw or the like. Reset shaft 114 extends upwardly through block 115 and its free end is arranged to be positioned beneath the bottom surface of lever 90. Normally, under the action of spring M3, the end of reset shaft M4 near lever 90 is retracted and spaced apart therefrom so that spring 94 is able to rotate lever 90 whereby the walls of aperture 93 may engage actuator button 78 within their grasp.

At the conclusion of a centrifuging run, and prior to the beginning of another run, reset button 105 is depressed manually causing reset shaft 114 momentarily to rotate reset lever 90 whereby actuator button 78 is released from the grasp of the walls of aperture 93, so that spring 83 returns said button to its original position.

When reset button lltS is released, spring M3 causes the retraction of reset shaft M4 so that its upper end becomes spaced apart from reset lever 90 to permit the latter to perform its function during a centrifuging run.

Although in both of the embodiments shown and described herein, the actuating elements have been illustrated as extending through an aperture in a lever, it is understood that the gripping action produced by the aperture walls upon the actuating element may also be produced by other portions of the lever, such as recess or other edges being urged against any suitable portion of the actuating element, whereby the latter is movable into and retained in successive gripped positions against the action of the primary spring acting upon said actuating element.

In the specification, I have explained the principles of my invention, and the best mo-de in which I have contemplated applying those principles, so as to distinguish my invention from other inventions; and I have particularly pointed out and distinctly claimed the part, mode or combination which I claim as my invention or discovery.

While I have shown and described certain preferred embodiments of my invention, it will be understood that modifications and changes may be made Without departing from the function and scope thereof, as will be clear to those skilled in the art.

What is claimed is:

l. In a switching device fora switch having an actuating element self-returning to an unactuated position, `an `overtravel mechanism for said switch, said overtravel mechanism comprising a first element mounted on said switch to move said actuating element from fan inoperative to an operative actuating position, first resilient means normally urging said first element apart from said actuating element, a lever pivotally mounted near said switch, an aperture in said lever through which said first element extends, second resilient means normally operative upon said lever to cause the walls of said aperture to assume a skewed position relative to and frictionally to engage the outer walls of said first clement and retain the latter in position against the action of said first resilient means.

2. ln a switching device for a switch having an actuating element self-returning to an unactuated position, an overtravel mechanism for said switch, said overtravel mechanism comp-rising a first means mounted on said switch and 'adapted to move said actuating element, from an inoperative to an operative actuating position and beyond said operative actuating position, first resilient means normally urging said first means lapart from said actuating element, second means adapted to engage said first means, second resilient means normally urging said second means into engagement with said first means and to maintain the latter in succeeding gripped positions to which said first means is moved step by step in a direction against the action of said first resilient means.

3. ln a switching device for a switch having an actuating element self-returning to an unactuated position, an overtravel mechanism for said switch, said overtravel mechanism comprising a first element mounted on said switch to move said actuating element from an inoperative to an operative actuating position, first resilient means normally urging said first element apart from said actuating element, a lever pivotally mounted near said switch, an aperture in said lever through which said first element extends, second resilient means normally operative upon said lever to cause the walls of said aperture to assume `a skewed position relative to and frictionally to engage the outer walls of said first element and retain the latter in position against the action of said first resilient means, said engagement between said aperture walls and said first element being less than sufficient to prevent said first element from being moved through Said aperture to succeeding engaged positions against the action of said first resilient means.

d. An actuating mechanism comprising a frame, a first element movable reciprocably relative to said frame, a first spring normally urging said first element in one direction relative to said frame, a lever pivotally mounted on said frame, an aperture in said lever through which said first element extends for reciprocable movement, a second spring positioned between said frame 'and said lever, said second spring normally urging said lever into a position where the `walls of said aperture exert a 1gripping action on said first element and retain the latter in position against the action of said first spring, the gripping laction between said aperture walls and said first element being less than sufficient to prevent said first element from being moved in the opposite direction through said aperture to succeeding gripped positions against the action of said first spring.

5. An actuating mechanism comprising a frame, an actuating element movable reciprocably relative to said frame, iirst means normally urging said element in one direction relative to said frame, second means pivotally mounted on said frame and arranged `to engage `said actuating element, third means operative upon said second means to cause the latter to grip said actuating element in various succeeding positions to which said element is moved in the opposite direction `against the action of said first means, said second means when pivoted against the action of said third means being opera* tive to release said actuating element and permit the latter to be moved freely in the first direction by said first means.

6. in a switching device for `a switch mounted on a frame and having an actuating element self-returning to an unactuated position, an overtravel mechanism for said switch, said overtravel mechanism comprising `a first element pivotally mounted on said switch, said first element movable toward and away from said actuating element, a first spring adapted normally to urge said first element away from said actuating element, a second element movable reciprocably relative to said frame, said second element being impinged upon by said first element normally to cause said second element to move in a first direction relative to said frame, a lever pivotally mounted on said frame, an aperture in said lever through which said second element moves reciprocably, a second spring mounted between said frame and said lever and normally urging said lever in one pivoting direction to cause the Walls of said aperture frictionally to engage said second element in succeeding gripped positions to which said second element is moved step by step in the opposite direction Iagainst the action of said rst spring.

7. In a -switching device for -a switch mounted on a frame and having an actuating element self-returning to an unactuated position, an overtravel mechanism for said switch, said overtravel mechanism comprising "a first element pivotally mounted on said switch, said first element movable toward and away from said actuating element, a first spring adapted normally to urge said rst element away from said actuating element, a second element movable reciprocably relative yto said frame, said second element being impinged upon by said first element norm-ally to cause said second element to move in a first direction relative to said frame, a lever pivotally mounted on said frame, an aperture in said lever through which said second element moves reciprocably, a second spring mounted between said frame and said lever and normally urging said lever in one pivoting direction to cause the walls of said aperture to engage said second element in succeeding gripped positions to which said second element is moved step by step in the opposite direction against the `action of said first spring, the movement of said lever against the action of said second spring in the opposite pivoting direction permitting said aperture walls to release said second element whereby the latter is caused by said first spring to move freely in said rst direction.

8. In a switching device for a switch having an actuating element self-returning to an unactuated position, an overtravel mechanism for said switch, said overtravel mechanism comprising a first element movable reciprocably relative to said actuating element, a first spring normally urging said first element apart from said actuating element, means limiting the movement of said first element apart from said actuating element, a second element adapted to engage said first element, a second spring normally urging said second element to engage said first element and maintain the latter in succeeding gripped positions to which said first element is moved step by step in a direction against the action of said first spring.

9. A device according to claim 8, and further comprising means for operating said second element against the action of said second spring to release said first element whereby the latter is moved freely under the action of said first spring.

10. In a switching device for a switch having an actuating element self-returning to an unactuated position, an overtravel mechanism for said switch, said overtravel mechanism comprising a frame mounted on said switch, an actuating button movable reciprocably within said frame opposite said actuating element, a first spring normally urging said button outwardly from said actuating element, a flange on said button, a shoulder on said frame cooperating with said fiange to limit the outward movement of said button, a lever pivotally mounted on said frame, an aperture in said lever, said button extending through said aperture, a second spring positioned between said frame and said lever normally acting to cause the walls of said aperture to engage said button in succeeding gripped positions to which said button is moved step by step toward said actuating element against the action of said first spring.

11. A device according to claim 10, and further comprising an instrument panel with which said switch is associated, an elongated shaft mounted between said panel and said lever, a third spring for normally retracting the end of said shaft from said lever, a reset button operative upon said shaft against the action of said third spring to cause the end of said shaft to move said lever against the action of said second spring, whereby said actuating button is released from said lever and is caused by said first spring to move apart from said actuating element freely under the action of said first spring.

l2. In a centrifuge system having a shaft rotatable by an electric motor and a switch operative to turn said motor on and off, an overtravel mechanism for protecting said switch comprising an actuating element on said switch, said actuating element being self-returning to an unactuated position, a first element movable reciprocably relative to said actuating element and adapted to move the latter toward an actuated position, a first spring normally urging said first element apart from said actuating element, a second element adapted to engage said first element, a second spring normally urging said second element into engagement with said first element and to maintain the latter in succeeding gripped positions to which said first element is moved step by step in a direction against the action of said first spring, said second element being intermittently engageable by said shaft and movable toward said first element by successively widening divergences of said shaft from its normal axis.

13. ln a centrifuge system having a shaft rotatable by an electric motor and a switch operative to turn said motor on and off, an overtravel mechanism for protecting said switch comprising an actuating element on said switch, said actuating element being self-returning to an unactuated position, first means movable reciprocably relative to said actuating element and adapted to move the latter toward an operative actuating position, first spring means normally urging said first means apart from said actuating element, second means adapted to engage said first means, second spring means normally urging said second means into engagement with said first means and to maintain the latter in succeeding gripped positions to which said first means is moved step by step in a direction against the action of said first spring, third means on said shaft adapted intermittently to engage said second means as said shaft diverges from its normal axis in successively widening paths, the dimensions of said first, second and third means being selected whereby said first means moves said actuating element to its actuating position upon a predetermined divergence of said shaft from its normal axis.

14. In a centrifuge system having a shaft rotatable by an electric motor and a switch operative to turn said motor on and off, an overtravel mechanism for protecting said switch comprising an actuating element 011 said switch, said actuating element being self-returning to an unactuated position, first means movable reciprocably relative to said actuating element and adapted to move the latter toward an operative actuating position and beyond said actuating position, first spring means normally urging said first means apart from said actuating element, second means adapted to engage said first means, second spring means normally urging said second means into engagement with said first means and to maintain the latter in succeeeding gripped positions to which said rst means is moved step by step in a direction against the action of said first spring, third means on said shaft adapted intermittently to engage said second means as said shaft diverges from its normal axis in successively widening paths, the dimensions of said first, second and third means being selected whereby said first means moves said actuating element to its actuating position upon a predetermined divergence of said shaft from its normal axis, said second means preventing said rst means from disengaging said actuating element when said rst means has caused said actuating element to move beyond its actuating position.

LReferences Cited in the le of this patent UNITED STATES PATENTS 2,624,464 Morrison Ian. 6, 1953 2,784,584 Worst Mar. 12, 1957 2,807,952 Bochan et al. Oct. 1, 1957 

